更新表而不用临时表中的空值替换原始表中已经存在的值

Question

我有一个名为“学生”的原始表，如下所示：

idstudent    studentname    studentclass
101          adam           A
102          brian          NULL
103          charlie        C 
104          danny          D

然后，我有一个临时表名“student_temp”，如下所示：

idstudent    studentname    studentclass
101          adam           A
102          brian          B
103          NULL           C
105          edward         E

如果我在 SQL Server Management Studio 2008 R2 上运行此查询：

USE [StudentDatabase]
GO

UPDATE stu
SET [studentname] = tmp.[studentname]
  , [studentclass] = tmp.[studentclass]

FROM [StudentDatabase].[dbo].[student] stu
INNER JOIN [StudentDatabase].[dbo].[student_temp] tmp  
    ON stu.[idstudent] = tmp.[idstudent]
GO

INSERT INTO student
SELECT 
    tmp.[idstudent]
  , tmp.[studentname]
  , tmp.[studentclass]

FROM [StudentDatabase].[dbo].[student] stu
RIGHT OUTER JOIN [StudentDatabase].[dbo].[student_temp] tmp 
    ON  stu.[idstudent] = tmp.[idstudent]
WHERE stu.[idstudent] IS NULL

'student'表的结果会是这样的：

idstudent    studentname    studentclass
101          adam           A
102          brian          B
103          NULL           C
104          danny          D
105          edward         E

但我想要的是，结果会是这样的：

idstudent    studentname    studentclass
101          adam           A
102          brian          B
103          charlie        C
104          danny          D
105          edward         E

注意idstudent=103 在student 表中已经有studentname 值“charlie”。但它被student_temp表中的记录替换为NULL。

有没有办法解决这个问题？

Answer 1

在WHERE 子句中使用条件忽略NULL 值

UPDATE stu
SET [studentname] = tmp.[studentname]
  , [studentclass] = tmp.[studentclass]

FROM [StudentDatabase].[dbo].[student] stu
INNER JOIN [StudentDatabase].[dbo].[student_temp] tmp  
    ON stu.[idstudent] = tmp.[idstudent]
WHERE tmp.[studentname] IS NOT NULL

Answer 2

这整个事情通常应该用MERGE和COALESCE来完成：

MERGE INTO student t
USING student_temp s
ON t.idstudent = s.idstudent
WHEN MATCHED THEN UPDATE SET
  studentname = COALESCE(t.studentname,s.studentname),
  studentclass = COALESCE(t.studentclass,s.studentclass)
WHEN NOT MATCHED THEN INSERT (idstudent,studentname,studentclass)
VALUES (s.idstudent,s.studentname,s.studentclass);

如果两个表都具有非空值，则当前有利于现有数据。如果您希望用 student_temp 中的新值覆盖，请反转 COALESCE 表达式中的值顺序。

所以，更喜欢来自student_temp 的值：

MERGE INTO student t
USING student_temp s
ON t.idstudent = s.idstudent
WHEN MATCHED THEN UPDATE SET
  studentname = COALESCE(s.studentname,t.studentname),
  studentclass = COALESCE(s.studentclass,t.studentclass)
WHEN NOT MATCHED THEN INSERT (idstudent,studentname,studentclass)
VALUES (s.idstudent,s.studentname,s.studentclass);

Answer 3

试试这个：

UPDATE stu
SET [studentname] = CASE WHEN tmp.[studentname] IS NOT NULL THEN tmp.[studentname] ELSE stu.studentname END
  , [studentclass] = CASE WHEN tmp.[studentclass] IS NOT NULL THEN tmp.[studentclass] ELSE stu.studentclass END

FROM [student] stu
INNER JOIN [student_temp] tmp  
    ON stu.[idstudent] = tmp.[idstudent]
GO

INSERT INTO student
SELECT 
    tmp.[idstudent]
  , tmp.[studentname]
  , tmp.[studentclass]

FROM [student] stu
RIGHT OUTER JOIN [student_temp] tmp 
    ON  stu.[idstudent] = tmp.[idstudent]
WHERE stu.[idstudent] IS NULL

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